1. Field of the Invention
The present invention relates to micromachined devices and, more particularly, to a coupling system for micromachined devices.
2. Description of Related Art
Recent advances in micromachining have enabled the manufacture of various microelectromechanical systems (MEMS) that offer potential performance and cost improvements over existing non-micromachined devices. MEMS devices may be manufactured on a large scale using photolithographic techniques to etch silicon wafers, in much the same way that traditional microelectronic integrated circuits are produced in the electronics industry. In silicon-based MEMS devices fabricated using conventional integrated circuit techniques, three-dimensional structures can be integrated with electronic circuitry on the same chip, offering great potential for improvements of sensors, actuators, and other devices. Initially, MEMS devices were strictly silicon-based, like microelectronic devices, but today the term represents complete miniature devices that may or may not be silicon-based, and that can be produced using methods other than photolithographic techniques.
One MEMS device is a micro-electromechanical system gyroscope (MEMS gyro). The MEMS gyro consists of one or more oscillating proof masses that may be suspended above a substrate by support arms mounted to the substrate. The proof masses are made to oscillate at a precise frequency axially and parallel to the substrate by an electronic drive mechanism. As used herein, the term “proof mass” is defined broadly to include any mass suitable for use in a MEMS system. The MEMS gyro functions by sensing the coriolis acceleration that acts on the oscillating proof masses when the gyro is rotated. Further, the substrate typically has a recess below the proof masses that allows the gyro to oscillate freely above the substrate. The recess may be formed in the substrate by deposition of a photoresist mask that allows the substrate to be selectively etched.
When spring elements are used to suspend a proof mass above a substrate, at least one end of the spring element is typically mounted to the substrate, and the other end is typically attached to the proof mass. Because one end is fixed, and also because micro-machined structures do not have pin joints, a spring element must typically stretch as well as bend when the proof mass oscillates axially. Adding spring elasticity to each of the spring elements used to suspend a proof mass can accommodate this stretching.
When proof masses are mounted so that their spring elements must stretch to allow movement, however, the resulting spring constants in the direction of oscillation are non-linear. Non-linear spring constants can introduce frequency shifts if the amplitude of the mass' oscillation varies. Such frequency shifts are undesirable, as they can affect the accuracy of a MEMS gyroscope. Moreover, the performance of any micromachined device that employs a movable mass may be adversely affected by a non-linear spring constant in the suspension system. Thus, a suspension system with a more linear spring constant could provide improved performance in micromachined devices.
In addition, suspending a proof mass with a spring element that is configured to stretch and also to bend as the proof mass oscillates allows some freedom of motion in directions other than the direction in which the proof mass was designed to oscillate. Such freedom of motion is undesirable, can adversely affect measurements made by the gyro and, if it is great enough, may even damage or destroy the gyro if a portion of the proof mass collides with a stationary element of the gyro. Thus, a suspension system that allows great freedom of motion along one axis, while significantly restricting motion in any other direction in the plane of the substrate may provide improved reliability and performance in micromachined devices.
Further, in a MEMS gyro, two proof masses may be used to improve the sensitivity of the device over those having just one proof mass. In such a configuration, the proof masses may be driven to oscillate 180° out-of-phase with each other, so that the coriolis acceleration created by an input rotation causes each proof mass to be displaced in a direction opposite from the other proof mass. The total displacement of both proof masses (which is twice the displacement of each single proof mass) may then be measured.
When two proof masses are used to form a MEMS gyro, it is desirable for them to oscillate out-of-phase (i.e., in synchronized opposition) so that the difference in displacement of the two proof masses may be used to increase the sensitivity as described above. If the oscillation of the two proof masses were, for example, in phase rather than in synchronized opposition, the force acting on one proof mass would cause displacement in the same direction as the other proof mass, leaving no differential displacement to measure.
Further, it is desirable for both proof masses, when coupled, to oscillate at a resonant frequency (i.e., at a “coupled frequency”) that occurs only when the masses oscillate in synchronous opposition, and that is also different from any other resonance of the system, such as the resonant frequency of both coupled proof masses oscillating in full synchronization. The resonant frequency of both coupled proof masses oscillating in full synchronization is very near the uncoupled resonant frequency of each proof mass, as will be described in more detail below.
Oscillation at a coupled frequency ensures that both proof masses oscillate at a desired frequency and in synchronous opposition whenever they are driven at a frequency that is near the coupled frequency. Moreover, oscillation at a coupled frequency allows the drive signal to be small, which decreases interference with the output signal of the gyro.
Ensuring that the two proof masses oscillate out-of-phase and at a discrete resonant frequency through the use of electronics, rather than by creating a coupled frequency as described, is difficult at best, and may greatly increase the complexity of the drive and control system of the MEMS gyro and may also decrease the performance of the gyro. Thus, a coupling system that helps to ensure out-of-phase operation at a discrete resonant frequency may simplify the design and improve the performance and reliability of a MEMS gyro.